This invention relates to electroacoustic transducers, such as microphones, and in particular to a structure which is incorporated into a semiconductor substrate.
Demand is growing for electroacoustic transducers which may be formed as part of a semiconductor integrated circuit. These transducers may include, for example, microphones incorporated into the circuitry of telecommunications and audio recording equipment, hearing aid microphones and receivers, or miniature speakers. In the case of microphones, electrostatic device technology presently in widespread use generally takes the form of a metalized polymeric foil (which may be charged) supported over a metalized backplate or stationary structure so as to form a variable capacitor responsive to voiceband frequencies. While adequate, such devices are relatively large, discrete components which cannot be integrated into the semiconductor integrated circuitry with which they are used.
Recently, such an integrated microphone structure and a method of manufacture were proposed. (See U.S. Patent application of I. J. Busch-Vishniac et al., Ser. No. 469,410, filed Feb. 24, 1983 and assigned to Bell Telephone Laboratories, which is incorporated by reference herein.) Briefly, the microphone included a tensioned membrane formed from a thinned portion of a thicker semiconductor substrate. The membrane had an area and thickness such that it vibrated in response to incident sound waves. A pair of electrodes formed a capacitor, with one of the electrodes vibrating with the membrane to vary the capacitance when a biasing voltage was applied and produce an electrical equivalent to the acoustic signal. It has also been suggested that an integrated capacitive microphone can include an insulating layer with fixed charge for providing a built-in diaphragm bias for the device. (See U.S. Patent application of W. S. Lindenberger, T. L. Poteat, and J. E. West, Case 2-2-24, filed on an even date herewith and assigned to Bell Telephone Laboratories.)
While such structures offer considerable promise for the replacement of the distinct microphones now in use, several problems and considerations remain in the commercial realization of an integrated microphone. Foremost, it is desirable to make the area of the vibrating element as small as possible to reduce cost. However, a small area tends to cause a drop in sonic force on the diaphragm element, thereby lowering the sensitivity of the device. Further, smaller area diaphragms produce a smaller device capacitance which in turn tends to increase the noise associated with on-chip circuitry coupled to the device and also tends to further decrease the integrated microphone sensitivity through capacitance divider action. In order to alleviate such effects of reduced area (i.e., reduced signal-to-noise ratio), it is desirable to reduce the stiffness of the diaphragm.
The above-noted effects of reduced diaphragm area may also be compensated for by a reduction in the thickness of the air gap between capacitor electrodes. We have found, however, that for air gaps below approximately 1.5 .mu.m with other dimensions optimized for certain telephone applications, the electrical output frequency response of a microphone with a tensioned diaphragm had a tendency to fall at an unacceptable rate with frequency when utilizing acoustic venting means, common in commercially available devices, comprising 4-20 holes around the periphery of the stationary electrode and backplate. That is, the devices would be overdamped, even at the critical 300-3,500 Hz portion of the audioband which is transmitted in telephone equipment. Specifically, with an air gap of 0.25 .mu.m which yielded the optimum signal-to-noise ratio, the response fell more than 20 dB across the telephone frequency band indicating severe overdamping. Sensitivity levels were also inoperably low. It is, therefore, also desirable to provide some acoustic venting means which will permit reduced area and produce an acceptable output signal at telephone band frequencies or other frequency bands of interest.
A reduction in the air gap thickness will also have another beneficial effect, which is to reduce the external dc voltage level needed to bias the diaphragm. This would provide an alternative to the requirement of a built-in diaphragm bias as suggested in the application of Lindenberger, previously cited.
In addition, a silicon integrated microphone will generally have a nonrising output response as a function of frequency in the audio bandwidth. In some applications, it may be desirable to tailor the response to provide a peak at a certain frequency by means of an appropriately shaped acoustic port and coupling cavity. Further, the microphone chip may, under certain circumstances, be subject to high electromagnetic interference (EMI), and so some shielding means may be needed. In the design of an integrated microphone, therefore, it is desirable to provide these functions with a minimum number of piece parts.
It is therefore an object of the invention to provide an integrated electroacoustic transducer with a small diaphragm area which still provides an acceptable frequency response and signal-to-noise ratio. It is a further object of the invention to provide an electroacoustic transducer which can be operated at a low dc bias. It is a still further object of the invention to provide acoustical interconnection and tuning means, and EMI shielding means, for an integrated electroacoustic transducer.